1. Field of the Invention
The present invention relates to control of a diesel engine.
2. Description of the Related Art
Conventionally, a technique has been proposed for controlling a diesel engine in a combustion form (mode) corresponding to the load of the engine (see, for example, Patent Document 1 listed below). Combustion forms (also called “combustion modes”) of fuel employed in such engine control include a diffusion combustion mode for combusting fuel while injecting the fuel into a combustion chamber, and a premixed combustion mode (also called a “homogeneous-charge compression combustion mode”) for mixing fuel and air within a combustion chamber before igniting the fuel. In the following description, the diffusion combustion mode and the premixed combustion mode are also referred to as “diffusion combustion” and “premixed combustion,” respectively.
In general, diffusion combustion is used when the engine is in a high-load state, and premixed combustion is used when the engine is in a low-load state. In premixed combustion, EGR control is concurrently used for re-circulating a large amount of exhaust gas to the intake side. It has been known that the amounts of NOx, soot, etc., can be reduced by switching the combustion mode to the premixed combustion mode.
During a transition period of switching the combustion mode, nitrogen oxide (NOx), soot (soot itself or opacity of exhaust gas), combustion noise, etc., may be generated or their amounts may increase. Examples of such a transition period of switching the combustion mode include a transition period of switching from diffusion combustion to premixed combustion, a transition period of switching from premixed combustion to diffusion combustion, and a transition period of switching from motoring to premixed combustion. In order to solve the above-described problems, Patent Document 2 listed below proposes to divide fuel injection into pilot injection and main injection, and to finely adjust an increase or decrease in fuel injection amount during the transition period.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2007-211612
[Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2010-236459
3. Problems to be Solved by the Invention
It has been known that when premixed combustion or diffusion combustion takes place in a diesel engine, fuel injection timing and EGR amount greatly affect the combustion. However, immediately after switching the combustion mode, the EGR amount cannot be controlled properly due to, for example, a temporal delay in introduction of exhaust gas. Heretofore, a parameter for properly controlling fuel injection timing during the transition period of switching the combustion mode has not been known.
Data from various types of sensors such as an intake pressure sensor, an air flow sensor, an exhaust oxygen sensor, and an intake temperature sensor are influenced by factors which are difficult to predict, such as fuel properties, changes in the cooling water temperature, and deterioration of various sensor types with time. Therefore, in the case where a predictive value of the EGR ratio or the oxygen concentration of intake gas is obtained using a model and based on sensor data and the fuel injection timing during the transition period of switching the combustion mode is controlled based on the predictive value, it is necessary to take measures such as providing initial control boundaries in consideration of these factors. As described above, even a method utilizing a model cannot accurately predict the oxygen concentration of intake gas, etc. Therefore, it has been difficult to improve the combustion state by increasing accuracy in controlling the fuel injection timing during the transition period of switching the combustion mode.